A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen, an alcohol such as methanol or ethanol, or formic acid, is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.
Electrocatalysts for fuel oxidation and oxygen reduction are typically based on platinum or platinum alloyed with one or more other metals. The platinum or platinum alloy catalyst can be in the form of unsupported nanometre sized particles (metal blacks) or can be deposited as even higher surface area particles onto a conductive carbon substrate (a supported catalyst).
Fuel cells are usually classified according to the nature of the electrolyte employed. In the PAFC, cells are fabricated from a phosphoric acid electrolyte contained in a thin inert matrix layer sandwiched between the anode and cathode electrodes. In the PEMFC, the electrolyte layer is typically a thin proton-conducting polymer located between the electrode layers. Either of these cells can operate on pure hydrogen fuel, or a more dilute hydrogen containing fuel mixture formed by the reforming of a hydrocarbon fuel, or particularly in the case of the PEMFC, can operate directly on hydrocarbon fuels such as methanol or ethanol.
The electrodes of the PAFC and PEMFC, often referred to as gas diffusion electrodes (GDE), usually comprise a gas-porous, electrically conductive and chemically inert gas diffusion substrate (GDS) and an electrocatalyst layer, comprising the electrocatalyst, which is facing, and in contact with, the electrolyte or membrane. Phosphoric acid fuel cells use phosphoric acid as the electrolyte and typically operate at a temperature of between 150° C. and 220° C. Within the research published or patented in the relevant area, alloying platinum with transition metals such as chromium, cobalt and nickel can boost the catalyst activity for the oxygen reduction reaction by a factor of 2 or 3 over platinum catalysts in PAFC systems. Conventional catalysts employed at the cathode of phosphoric acid fuel cells are platinum-chromium (U.S. Pat. No. 4,316,944), platinum-nickel (U.S. Pat. No. 5,068,161) and platinum-cobalt-chromium (U.S. Pat. No. 4,447,506) A problem associated with such catalysts is a lack of stability due to sintering of the catalyst at the temperatures used in phosphoric acid fuel cells. Catalyst particles have a tendency to coalesce either by surface migration or dissolution/reprecipitation during prolonged operation in the phosphoric acid fuel cell environment. This leads to an unacceptable reduction in cell performance (voltage) over the lifetime of the fuel cell system and a concomitant reduction in the efficiency of the fuel cell's electrical generating capability. Therefore the search continues for oxygen reduction catalysts with higher activity and stability. Ternary systems have been shown to be more stable over binary compositions and the stability in a phosphoric acid fuel cell environment can be improved by the addition of a third metal. U.S. Pat. No. 4,806,515 describes a ternary system of platinum-chromium-gallium with substantial stability over sintering and resistance to metal dissolution. Platinum-cobalt-chromium (U.S. Pat. No. 5,068,161) and platinum-rhodium-iron (WO94/10715) have also demonstrated some enhancement of stability in the phosphoric acid fuel cell environment as have platinum-iridium-chromium and platinum-iridium-iron (U.S. Pat. No. 5,013,618).